1. Field of the Invention
This invention relates generally to monitoring of usage of aircraft parts, systems and functions, and more particularly concerns monitoring of aircraft braking systems.
2. Description of Related Art
Automatic braking systems have been commonly provided on commercial aircraft to aid the deceleration of the aircraft upon landing. As the size and complexity of aircraft have increased, the automatic braking systems have also become more complex and computerized. Modern anti-skid systems incorporated into aircraft braking systems commonly optimize braking efficiency by adapting to runway conditions and other factors which affect braking in order to optimize deceleration, typically corresponding to the level of brake pressure selected by the pilot.
In a conventional skid detection system used in aircraft braking systems typically includes a wheel speed transducer for each wheel brake of the wheels of the aircraft, for measuring wheel speed and generating wheel speed signals that are a function of the rotational speed of the brake wheel. The wheel speed signal is typically converted to a signal representing the velocity of the aircraft, and compared with a desired reference velocity, to generate wheel velocity error signals indicative of the difference between the wheel velocity signals from each braked wheel and the reference velocity signal for providing anti-skid control of aircraft braking.
Aircraft brakes often constitute the most expensive single maintenance item to the airlines, because they wear out and are expensive to replace. The demands on aircraft brakes are extraordinary due to the high speeds at which aircraft braking typically occurs, causing aircraft brakes to periodically wear out and require replacement. In emergency braking situations, reverse thrust may not be available, high performance engines can take several seconds to reduce thrust, and flight control surfaces may be able to generate little aerodynamic drag, placing the burden of slowing the aircraft almost completely on the aircraft braking system. In such situations, although the aircraft wheel size maybe comparable to that of an average car, the amount of energy that must be absorbed per brake can be equivalent to what a car brake would absorb completely stopping an average car from a speed of 60 miles an hour 200 times.
Aircraft brakes are typically have a brake stack formed of multiple disks, in which the key elements are the rotors and stators which absorb the energy of a stop. The rotors are keyed to and rotate along with the wheels, while the stators, tied to the axle, are stationary. Hydraulically operated brake pistons compress the rotors and stators together to provide the frictional forces necessary to brake an aircraft. One or more metal wear pins are attached to the pressure plate, or first stator, of a brake stack, and extend through a hole in the brake's actuator housing. When the brake is new, the pins extend past the housing by some amount (e.g. one or two inches). As the brake wears, the pressure plate moves away from the brake actuator housing, and the wear pins move with the pressure plates. The stroke of the brake pistons is automatically adjusted by brake actuator adjusters based upon the brake wear indicated by the wear pins, so that when the brakes are released, the actuators only retract a fixed small amount, independent of brake wear.
Aircraft brakes are commonly guaranteed for a given number of landings, and the number of landings is commonly used as the overall measure of brake usage. However, the rate at which aircraft brakes wear and require replacement, and hence the cost of maintenance of aircraft brakes, is heavily dependent upon how such aircraft brakes are used. Factors such as aircraft wheel speed and brake pressure, as well as how an operator applies brakes during each flight cycle can greatly affect aircraft brake wear. There thus remains a need for an improvement in the monitoring of aircraft brake usage. Improved aircraft brake usage monitoring would provide a better basis for guaranteeing brakes, and would provide valuable data to help the airline operator improve landing and braking procedures to achieve lower brake costs. The present invention meets these needs.